NavList:

Jim Martin, you wrote:

"According to "Nicholls concise guide" vol.1, a close by object
or straight line should not be used to determine index error,because it
does not allow for sextant parallax and atmospheric refraction. I can
give the textbook explanation if it is of interest."

 

Sextant parallax, yes. That's an important issue for index correction. But refraction, no. The only time terrestrial refraction would make a difference is if you're comparing sight lines to two objects at distances differing by a good fraction of a mile. I can't think of any instance where this would matter for standard index correction observations.